1. Field of Invention
The present invention relates to a fabrication method of integrated circuits. More particularly, the present invention relates to a fabrication method of a p-type channel field-effect transistor (FET).
2. Description of Related Art
The demand for high speed and low power consumption for logic devices can be accomplished by miniaturizing transistors. Scaling transistors to smaller dimensions can provide the logic devices with sufficiently high transistor saturation drain current and low gate capacitance for high speed and reduce leakage current for low power consumption.
However, as the size of a transistor is further reduced, various problems generated from the short-channel effects become significant. The ultra-shallow junction formation technique is one method used to resolve the short-channel effects. According to the traditional ultra-shallow junction formation technique, after the formation of a gate electrode, dopants are implanted with an appropriate amount of energy to two sides of the gate electrode, followed by performing rapid thermal annealing to generate the junction profile. Before the 90 nanometer (nm) generation, achieving the propoer resistance and depth basically relies on lowering the implantation energy of dopants and diminishing the annealing time. However, after the arrival of the 65 and 45 nm generations, the conventional technique is no longer applicable. Co-implantation, laser annealing and high-angle ion implantation techniques are being investigated.
The concept of co-implantation technique is based on the fact that ion implantation causes interstitials injection. These interstitials are routes for transient enhanced diffusion of boron ions during spike annealing. The co-implantation schemes have shown to improve such an effect because the species implanted by co-implantation form bonds with the interstitials. Ultimately, the transient enhanced diffusion of boron ions and the formation of boron cluster caused by the interstitials are reduced.
Currently, carbon is the most commonly employed species in a single co-implantation process for increasing saturation voltage and for controlling the short-channel effects. However, the implanted carbon ions create abrupt junction depth profile. Ultimately, high electric field is resulted that in turns induces serious current leakage, especially at the side-wall-gate.
Fluorine ions are also of great interest as species for a single co-implantation process because both the depth and the abruptness of the junction profile can be better controlled. Ultimately, current leakage is mitigated. However, co-implantation with fluorine ions provides limited improvement on the saturation voltage. Hence, it is ineffective in improving the short-channel effects.
Another approach for enhancing the effectiveness of a device is by altering the mobility of the source/drain region. Since the traveling speed of electrons and holes in a silicon channel is limited, the application of this approach in transistors is also limited. The technology of employing a silicon germanium material for the source/drain region of a transistor has been proposed. This technology basically includes removing a portion of the silicon substrate pre-determined for forming the source/drain region, followed by employing the selective epitaxial technology to re-fill the substrate with silicon germanium. Comparing a source/drain region formed mainly with a silicon germanium material with that formed with a silicon material, germanium has a smaller electron effective mass and hole effective mass, the source/drain region formed with silicon germanium can enhance the mobility of electrons and holes. As a result, the effectiveness of the device is improved.
However, during the formation of silicon germanium, the epitaxy growth process is conducted at extremely high temperature. The heat provided for the formation of silicon germanium also causes the diffusion of boron, which ultimately leads to the short-channel effects.